Re: [RFC PATCH v2 4/9] iommu/arm-smmu-v3-sva: Allocate new ASID from installed_smmus

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Aug 22, 2023 at 06:57:00PM +0800, Michael Shavit wrote:
> Pick an ASID that is within the supported range of all SMMUs that the
> domain is installed to.
> 
> Signed-off-by: Michael Shavit <mshavit@google.com>
> ---
> 
> (no changes since v1)
> 
>  .../iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c   | 23 +++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
> index fe88a7880ad57..92d2f8c4e90a8 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
> @@ -66,6 +66,20 @@ static int arm_smmu_write_ctx_desc_devices(struct arm_smmu_domain *smmu_domain,
>  	return ret;
>  }
>  
> +static u32 arm_smmu_domain_max_asid_bits(struct arm_smmu_domain *smmu_domain)
> +{
> +	struct arm_smmu_master *master;
> +	unsigned long flags;
> +	u32 asid_bits = 16;
> +
> +	spin_lock_irqsave(&smmu_domain->devices_lock, flags);
> +	list_for_each_entry(master, &smmu_domain->devices,
> +			    domain_head)
> +		asid_bits = min(asid_bits, master->smmu->asid_bits);
> +	spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);
> +	return asid_bits;
> +}

I still don't like this, it is not locked properly. You release the
devices_lock which means the max_asid could change before we get to
arm_smmu_write_ctx_desc()

If you want to take this shortcut temporarily then a global max_asid
is probably a better plan. Change it to a per-master allocation later
to remove that.

Jason